Plasma etching systems have included reactor chambers including a pair of electrodes to which R.F. power is applied. A wafer including a film thereon to be etched is generally placed on one of the electrodes. Suitable gases are injected into the chambers and plasma is formed to provide the etching of the film. High or low radio frequencies are used in the etching process dependent upon the type of film being etched.
One of the problems found in R.F. plasma etching involves excessive stray electrical discharges from the electrodes to the reactor chamber and other parts in the system. When the wafer is on a grounded chuck electrode and the voltage is applied to an upper counter electrode, the voltage between the upper electrode and the wafer is always less than the voltage between the upper electrode and the walls of the system which are generally grounded. This is because the wafer itself is never exactly at chuck potential, but is electrically isolated from the chuck electrode by an insulating coating on the back of the wafer. The result is that some of the current goes from the upper electrode to various grounded surfaces in the system instead of to the wafer where it will do some good in the etching process.
The higher the power the worse the problem of stray discharges becomes, and the more R.F. current is diverted to grounded surfaces. The stray discharges become a problem because a certain amount of power still has to reach the wafer. Stray discharges tend to be erratic, and are unstable. This means that it is difficult to always predict how much power will go to the wafer and how much will go into stray discharges.